This invention relates in general to an electronically controlled hydraulic brake actuation system for automobiles, and more specifically to a brake by wire system in which the vehicle wheel brakes are not normally hydraulically connected to the brake pedal. In such a system, brake pedal feel (pedal travel versus pedal force characteristics) is normally independent of wheel brake characteristics.
In conventional hydraulic vehicle brake systems, the force used to pressurize hydraulic fluid to operate the vehicle brakes comes from the vehicle operator pressing on the vehicle brake pedal to actuate the master cylinder of the brake systems. Normally, the resultant movement of the brake pedal linkage to the master cylinder is also used to actuate a vacuum or hydraulic boost system to provide an assisting for which aids the force provided by the vehicle operator in actuating the master cylinder and thus operate the vehicle brakes.
In certain recent advanced vehicle brake systems, such as so-called brake-by-wire brake systems, it is known to supply all of the force to operate the vehicle brakes from mechanical devices such as pumps or electrical linear actuators. When the mechanical device is a pump, the pressure generated by the pump and applied to the wheel brake will frequently be regulated by valves interposed between the pump and the wheel brakes. An example of such a brake-by-wire system is described in German Patent Application DE 44 13 579 A1.
In some known brake-by-wire systems, the vehicle brake pedal is connected to sensors providing signals used to control the operation of these mechanical devices supplying the force for operating the vehicle brakes. Such sensors typically measure the amount of movement of the brake pedal, the force with which the operator steps on the brake pedal, or both. Since the brake pedal does not actuate the vehicle brakes, it is common to provide a pedal simulator (also known as a compliance unit) to provide a simulated load on the brake pedal which provides a reaction force to the operation of the brake pedal similar to that experienced during the operation of a brake pedal in a conventional brake system. By providing a simulated load on the brake pedal similar to that experienced during the operation of a brake pedal in a conventional brake system, the transition from conventional brake systems to advanced brake systems having a pedal simulator is eased, since the driver of a vehicle with such an advanced brake system will enjoy a "normal" pedal feel. An additional purpose of the simulated load on a brake pedal is to provide a reaction force to the operation of the brake pedal to enable the use of a pedal force sensor as an input to the control of the operation of the mechanical devices supplying the force for operating the vehicle brakes.